1. Technical Field
This invention generally relates to an improved mortar mixing drum, and in particular to a mortar mixing drum fabricated from polyethylene crosslink with a removable wear resistant plastic liner and paddles, and an elemental perimeter drum frame for rigidity.
2. Background Art
Present day mortar mixers are constructed of all steel. Typically, they have a cylindrically shaped drum open along the top side and a plurality of metal mixing paddles and wipers axially and rotatably secured within the drum. A rotation means, such as a gasoline engine, is provided to rotate a paddle axle to facilitate mixing the mortar. The ends of the paddles are provided with rubber wipers which contact the metal interior surface of the drum. This is necessary because the mortar sticks to the inside of the drum, as it also sticks to the paddles. It should be obvious that the life span of the rubber wipers is limited and they require frequent replacement.
Often, due to improper maintenance and cleaning, residual mortar is allowed to dry within the mixing drum and other surfaces, including the paddles. Because dried mortar is very difficult to remove from a metal surface, workers often resort to breaking the mortar loose with a hammer or other heavy object. Consequently, mortar mixers typically spend a great deal of time in the shop for repairs.
A similar problem exists in the analogous art of cement mixers. Attempts have been made to solve this problem, such as Bishop's U.S. Pat. Nos. 4,435,082, 4,491,415, and 4,756,623, which all teach rotatable cement mixing drums manufactured of a plastic such as polyethylene. Adsit, U.S. Pat. No. 4,521,116, teaches a mixing apparatus having a removable and disposable synthetic resin drum liner. Kennedy, U.S. Pat. No. 4,711,582 teaches a rotary mixing device which uses a bag as a disposable drum liner. Riederer, U.S. Pat. No. 4,569,648 teaches a self-cleaning rotating drum being lined with elastic webs.
None of the above described devices are capable of mixing mortar, as they are designed for mixing cement. There is a significant difference between compositions of mortar and those of cement. Typically cement contains large gravel particles as compared to mortar whose largest component is relatively small grained lime cement and sand. There may be a substantial difference in weight, with standard cement gravel mixtures weighing approximately 150 pounds per cubic foot, whereas mortar will typically weigh approximately 200 pounds per cubic foot. As a result the polyethylene drums taught by Bishop's U.S. Patents and the rest of the cement mixers taught by the above-described prior art are not suitable for mixing mortar.
To date the most effective apparatus for mixing mortar uses a cylindrical drum having a cylindrical segment opening along the longitudinal, or elemental, length of the drum and a plurality of paddles rotatably secured within the drum. The ends of the paddles are provided with rubber wipers which wipe the inner surface of the cylindrical drum, keeping it free of adhering mortar. Contrary to their cement mixing counterparts, mortar mixers are not easily operated by hand, primarily because of the added density of the material being mixed, and must use either gasoline or electric motor to operate the mixing paddles.
The drum itself is rotatable from an orientation where the elemental segment opening is at the top to a dumping position where the opening is rotated downward until mortar is free to spill from the drum. Since the drum contains rotating paddles it presents a safety hazard, and the typical mortar mixer design includes protective grates covering the opening. Typically there are two sections to the protective grate, the first being fixed and the second hinged to open when mortar is being dumped from the drum. The grates are normally made of steel and have sufficient opening size to allow water and dry mortar to be easily dumped into the drum, yet small enough to keep operators' hands and clothing from being caught by the rotating mixing paddles.
Two problems plague both the manufacturers and users of such mortar mixers, the first is the abrasion caused by mortar being wiped against the inside walls of the mixing drum which will eventually cause the drum to wear through. The second is that mixed mortar will adhere to grates even during proper operation and must be chiseled off, and on occasion, mortar will be allowed to dry on the inside of the drum and the paddles to which it readily adheres. The only effective means of removing dried mortar from the inside of the drum and off the grates and paddles is to break it loose with a hammer or other heavy object. This is a time consuming chore and can easily result in damaged paddles and dents in the drum which interfere with the contoured surface of the drum and, if severe enough, will interfere with the rotation of the mixing paddles causing the paddle assembly to jam inside the drum. In practice it has been found that when a mortar mixer is being used on a full-time basis, the all steel drum should be replaced every three to six months. This of course is time consuming and expensive.
The use of plastic drums would be preferable since plastic such as polyethylene crosslink present a surface to which dried mortar will not adhere. The problems with the use of polyethylene crosslink are twofold, the first is wear on the inside of the drum caused by the abrasion of mortar being mixed, and the second is the considerable weight and high viscosity of mortar. The typical mortar mixer is designed to mix between six and eight cubic feet of mortar at a time. This can weight between 800 to 1,600 pounds.
The abrasiveness and high viscosity of mortar results in torque moments being imparted to the drum when the paddles are being rotated to mix mortar. These forces imparted can be considerable and can result in net torsional forces being imparted to the drum as a result of the means by which the drum is held in its upright or mixing position. These forces are the primary reason why steel has been the material of choice for fabricating mortar mixing drums.
As a result plastic mortar mixing drums had to be fabricated of thick, and relatively heavy, plastic materials.
What is needed is a plastic mortar mixing drum which has the combination of the non-adhering surfaces for the drum, the paddles and the grates, the structural strength and integrity of a steel drum, and an improved wear surface. The fabrication of such a drum would substantially increase its useful life and decrease the maintenance expenses incurred in removing dried mortar from the mixer parts and for periodically replacing mixing drums and paddles.